1. Field of the Invention
This invention relates to a novel lensmeter including a novel displaying system for displaying, as a number of digits, each of the ophthalmic properties of spectacle lenses, such as the back vertex refractive power (hereinafter referred to as "power") of a spherical lens, and the powers of the sphere and cylinder, and the direction of the axis of the cylinder of an astigmatic lens.
2. Description of the Prior Art
Lensmeters are the instruments for determining, by direct measurement, powers of spectacle lenses. Conventional lens-meters have generally the optical system as is shown in FIG. 1. In such a system, the pencil of light from lamp L passes through filter F1, target T having a dotted circle or corona C1 at the center as shown in FIG. 2, and through collimeter lens L1 which makes the pencil of light parallel. The pencil of parallel light enters via glass window G1 into a telescope composed of objective lens L3, focus plates S2 combined with angular notation plate S1, and eye lens L4, to focus on plate S2, so that the clear enlarged image of corona C1 of target T focused at the surface of focus plate S2 by objective lens L3 can be observed together with the angular notation of S1 through eye lens L4 of the telescope.
When test-lens L2 is inserted into set frame A in the system, the parallel pencil of light from collimeter lens L1 is refracted by the power of test-lens L2, so that it will be no longer parallel but convergent or divergent. As the result, no image of corona C1 is focused on focus plate S2. Then, target T is moved forwards or backwards along the optic axis so that the pencil of light which emerges from corona C1 and passes through test-lens L2 is again parallel and thus brought again into focus on plate S2, in which a clear image of corona C1 can be again observed. The said excursion of the target corresponds to the power of the sphere of the test-lens.
Conventional lensmeters also have another system as shown in FIG. 1. The system, which serves to convert the excursion of the target to a dioptric indication as shown in the lower portion of FIG. 3, consists of scale plate G2 (FIG. 1) which is movable in connection with target T or collimeter lens L1 and has a dioptric scale corresponding to the power of test-lens L2, index I for denoting a zero point and giving a small circular view as shown in the lower half of FIG. 3, and a device for making the clear image of both of the dioptric scale and the zero point on plate S2, the device consisting of prism P1, lens L5, filter F2, prism P2, lens L6, and prism P3.
It is clear that in a spherical lens when all the meridians of each surface have the same curvature, an image can at least theoretically be formed at a point, whereby corona C1 of target T can be focused on plates S2 to form an image of a similar dotted circle C2 as shown in FIG. 3, that is, a spherical lens can have only one power of the sphere which is independent of the rotation of the test-lens itself.
Unlike spherical lenses, such a phenomenon does not occur in the case of astigmatic lenses, because astigmatic lenses do not always have the same curvature in all the meridians of their surface. Those lenses may be of two types, cylindrical and toric. In the former its horizontal meridian is curved, but its vertical meridian is straight, while in the latter both its meridians are curved, but to a different degree, the vertical meridian being more curved than the horizontal. Where the two meridians in question are at right angles to each other, the condition is termed regular astigmatism; in ophthalmology most astigmatic lenses are regular.
Therefore, it is evident that the more curved meridian will refract the rays incident upon it to a greater degree than the less curved meridian so that, if parallel rays fall upon it, the vertical rays will come to a focus before the horizontal. There are thus two foci. When the vertical rays come to a focus while the horizontal are still converging, the focus is called the first focus, and when the horizontal rays come to a focus, the focus is called the second focus. The image at the both foci will be no longer a dotted circle as seen in FIG. 3, but be a cylinder composed of straight lines. Assuming that the image of corona C1 at the first focus is the image as seen in FIG. 4, the image of the corona at the second focus will be another image as seen in FIG. 5. The direction of the straight lines composing each image will be at right angles to each other. In astigmatic lenses, the dioptral value read at the first focus is generally termed the power of the sphere, denoted as (S), and the dioptral difference value read in going from the first focus to the second focus is termed the power of the cylinder, denoted as (C). The direction of axis of the cylinder is denoted as (Ax), and is the counterclockwise angle formed by the intersection of the direction of straight lines of the cylindrical image and the horizontal line of cross lines of the plate S1, provided that the astigmatic lens is horizontal in the lensmeter.
In a practical procedure for determining (S) and (C) of an astigmatic lens, no matter whether it is concave or convex, by using a conventional lensmeter as shown in FIG. 1, the dioptral power (S1) at a focus is initially determined and the dioptral power (S2) at another focus is then determined, and if the dioptral power (S1) is assumed to be (S), then (S1) minus (S2) is defined as (C). Unfortunately both the absolute value and/or the sign of (S) may vary. Also, the sign of (C) will vary depending on which focus of the two foci is first selected to determine the dioptic power of the lens. In addition to this fact, the format for indicating (S) and (C) differs from country to country. For example, in the United States of America (S) and (C) are indicated by (S) having a positive sign and (C) having a negative sign; in Europe with both having a positive sign; and in Japan with (S) having the same sign as (C) (+or -) if possible, or otherwise (C) having a negative sign. Accordingly, in the determination of (S) and (C) of convex or concave astigmatic lenses using one of the conventional lensmeters as shown in FIG. 1, it is required that the dioptral power (S1) is first determined at a certain focus; another dioptral power (S2) is determined at another focus; and then the dioptral power (S1') and (S2') are determined in inverse order, i.e., firstly at the second focus and secondly at the first focus, where (S1') being equal to (S2), and (S2') being equal to (S1). This transportation of the focus is termed the "(C) Sign Transposition" or "Cylinder For Transposition" value; thereafer the values (S) and (C) according to the customary format, which differs from country to country, are calculated from (S1), (S2), (S1') and (S2'). However such determinations, and especially such calculations, are not simple, but intricate.